The digital filter is well known and used in a variety of applications. In one application, a digital filter equalizes the variable attenuation of a voice-frequency message signal that has been transmitted on a telecommunication line. Every voice-frequency message signal comprises a plurality of single-frequency component signals. Generally, the amount of component signal attenuation varies significantly over the frequency spectrum, and to compensate for this variable attenuation, an inverse filter is selected that ideally has an amplitude-frequency response inverse to that of the line. Thus, the inverse filter and the line together should form an all-pass network, where the magnitudes of the frequency components of a message signal are attenuated equally across a desired frequency spectrum.
To determine the parameters of an inverse filter that equalizes the variable attenuation of a signal on a particular line, one prior art approach first involves measuring the attenuation of a signal at three different frequencies. From these measurements, the parameters of an inverse filter are calculated. These parameters should define an inverse filter that has a typical amplitude-frequency response and that equalizes the attenuation of the signal at the three measured frequencies. The problem with this approach is that the amplitude-frequency response of the line is assumed to be typical. However, when the amplitude-frequency response of the line varies unpredictably, this approach only equalizes the attenuation of a signal at the three measured frequencies with unacceptable variations of signal attenuation throughout the remainder of the frequency spectrum.
In applications requiring real-time determination of inverse filter parameters, the approach may be simplified by choosing the best filter from a predetermined set for a collection of typical transmission lines. This second approach has the same problem as the first approach and is less effective than the first for assuring that the variable attenuation of a message signal is equalized throughout a desired frequency spectrum.